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Carbon budgets - digging into the numbers

The rise in global average temperatures will predominantly be determined by the total (cumulative) emissions from the start of the industrial revolution and onwards.[1] This means that we have a set “carbon budget” of carbon dioxide emissions that can be emitted in order to have a given chance to stay within a certain temperature target.

The IPCC estimates that in order to have a 66% chance to meet the 2°C target, the total emissions of CO2 emitted from 1870 onwards need to remain below 2900 billion tons.[2] Roughly half this budget was consumed between 1870 and 2000.[3] In the 15 years between 2000 and 2014, more than one third of the remaining carbon budget from 2000 was consumed.[4] The remaining carbon budget from 2015 onwards (roughly 890 billion tons of CO2) will be consumed in less than 30 years if emissions are at constant 2014 levels.[5]

The longer we wait to start cutting emissions, the more rapid the rate of emissions reductions needs to be in order to stay within the budget. The Intergovernmental Panel on Climate Change (IPCC) puts it:

Any given level of warming is associated with a range of cumulative CO2 emissions, and therefore, e.g., higher emissions in earlier decades imply lower emissions later. [6]

It is also important to note that the remaining budget is significantly lower than the size of the proven fossil fuel reserves. For example, the International Energy Agency (IEA) observes that:

No more than one-third of proven reserves of fossil fuels can be consumed prior to 2050 if the world is to achieve the 2°C goal, unless carbon capture and storage (CCS) technology is widely deployed. [7]